1. Field of the Invention
The present invention relates to an optical disk drive, and more particularly an optical disk drive which is capable of writing, reading and erasing data.
2. Description of the Related Art
An optical disk drive records and erases data by focusing a laser beam onto a recording surface of an optical disk medium with an objective lens, heating a magnetic thin film on the recording surface to Curie point or a higher temperature and magnetizing a minute area irradiated with the laser beam by an external bias magnetic field in directions which are reverse between in a writing mode and an erasing mode. Therefore, it is requisite for the optical disk drive to form external bias magnetic fields, which must be magnetized in directions reverse to each other between the writing mode and the erasing mode.
To produce a bias magnetic field in a conventional optical disk drive, Japanese Patent Application Laid-Open No. 3-152702 discloses a configuration wherein a permanent magnet 1a is opposed to an optical head with an optical disk medium interposed therebetween as shown in FIG. 7. A coil 15 is disposed around the permanent magnet 1a as shown in FIG. 7 and a bias magnetic field required for writing or erasing is produced by rotating the permanent magnet 1a with a repulsive force or an attractive force acting between a magnetic field produced by supplying a current to the coil 15 and a magnetic flux from the permanent magnet 1a.
The configuration which rotates the permanent magnet as described above, however, poses a problem that it requires a large number of parts such as those of a magnet rotating mechanism and a space for rotating the permanent magnet, thereby enlarging an upper portion of the optical disk medium. Further, the configuration requires a long time for mechanically reversing the permanent magnet, or switching from the writing mode to the erasing mode and vice versa, thereby constituting a hindrance to high speed processing.
In order to solve these problems, there has been proposed a technique which produces bias magnetic fields only with a coil, instead of a permanent magnet. This technique utilizes magnetic fields which are produced by supplying a current to the coil as the bias magnetic fields. This technique allows magnetic fields to be produced in directions reverse to each other by changing a direction of the current dependently on operating modes. This technique makes it possible to configure an optical disk drive more compact since it requires no space for rotating a coil and parts in a number smaller than that of the parts required for the configuration which rotates the permanent magnet. However, this technique poses a problem that it requires a circuit which reverses a direction of an electric current as a circuit for driving a bias coil and another problem that a constant current must be supplied as in each of the writing mode and the erasing mode, thereby resulting in enhancement of power consumption and temperature rise in an optical disk drive.
A first problem is that the conventional technique inevitably requires reversing a direction of a produced magnetic field as described above whether it uses a permanent magnet or a coil as magnetic field producer. Therefore, it requires a mechanism which rotates a magnet when a magnet is used or a circuit which reverses an applied current in a driving circuit when a coil is used for a reason described below:
In an optical disk drive in which data is written as a direction of magnetization on a surface of an optical disk medium, data is written, erased and read by detecting a digital signal of 0 or 1 dependently on a variation of Kerr rotating angle resulting from the direction of magnetization. Therefore, it is requisite to apply external bias magnetic fields to the minute area onto which the laser beam is focused in directions which are reverse to each other dependently on the data writing and erasing modes.
A second problem is that the conventional technique requires continuously supplying a current to the coil in both the writing and erasing modes when a coil is used for producing the bias magnetic fields in particular, thereby resulting in the enhancement of power consumption and the temperature rise in the optical disk drive. The temperature rise in the optical disk drive hinders it from operating stably and degrading its characteristics for a reason described below:
Since it is requisite to produce the bias magnetic fields for the optical disk drive as described above, the bias magnetic fields must be produced by applying the constant current in both the writing and erasing modes when a coil is used as bias magnetic field producing means. Accordingly, the current is always consumed to produce the bias magnetic fields in the writing and erasing modes, thereby raising temperature in the optical disk drive.